Because of the substantial hardware complexity and weight penalties, plus aperture blockage, associated with the use of large sized antenna structures, such as parabolic reflector antennas, communication system users are increasingly turning to reduced mass, phased array antennas for high gain, large aperture applications, such as power-limited, satellite communication terminals. Where the antenna is to be deployed in a what is commonly termed a ‘comm on the move’ system, such as a land vehicle-mounted or shipboard communication system, there are often practical spatial limitations on the geometric layout of the array and thereby impact the performance of the antenna, especially where hemispherical coverage is required.
For example, as shown in the reduced complexity multi-panel array configuration of FIG. 1, a typical omni-directional coverage architecture places a plurality (three in the illustration) of antenna panel arrays 11, 12 and 13 in mutually orthogonal (X,Y,Z) planes, to allow one or more of the panel arrays to ‘see’ the signal of interest, irrespective of the orientation of the antenna relative to the direction of incidence of the signal. Each panel array contains a plurality of antenna elements spatially distributed on a planar surface and electronically controlled to form a prescribed beam pattern.
The gain for a signal along the boresight of the beam will depend upon the number of elements that make up the panel array, the gain of an individual element, and the direction of the beam (corresponding to the angle of incidence of the signal on the panel.) Maximum gain for a respective panel array occurs when the incidence angle is perpendicular to the array; the gain goes to zero when the incidence angle is zero or parallel to the surface of the panel array. The shape of the gain pattern is function of the array design. For signals that can be ‘seen’ by each of the three array panels 11, 12, 13 in the architecture of FIG. 1, the gain that can be obtained by combining each panel's contribution can be made the same as the maximum gain (e.g., raised cosine) for an individual panel array.
As diagrammatically illustrated in FIG. 2, in a conventional communication terminal that employs a gimballed (parabolic) dish 30, the output of the antenna feed, as received by a low noise amplifier (LNA) 31, is downconverted in a downconverter 33 to produce an IF signal, which is then applied to a demodulator 35 to derive the information signal (e.g., respective bits of a digitally encoded signal). These derived bits may then be subjected to intermediate processing (e.g., de-interleaving, decoding, decryption, demultiplexing, and the like) before being forwarded to the user. Where the terminal employs a multi-panel phased array, such as that shown in FIG. 1, described above, each panel array effectively operates as a separate antenna, so that multiple RF signals from the respectively different panel arrays 11, 12 and 13 are processed in combination to derive the information signal. This means that the performance of a multi-panel array is governed by the effectiveness of combined processing of the panel array outputs.
One relatively straightforward approach, diagrammatically shown in FIG. 3, would be to insert a switch 40 between the output of each panel and the downstream signal processing circuitry, and then couple the signal processing circuitry to whatever panel currently has the ‘best’ view of the signal (i.e., the panel array for which the angle of incidence of the signal is largest (closest to 90°). To compensate for discontinuities associated with switching from panel to panel, it would be necessary to employ some form of ‘handover’ mechanism for the demodulator/bit synchronizer, which could be relatively complex and thus significantly offset the apparent simplicity of the ‘switched’ approach.
In addition, as the multi-panel orientation is varied relative to the signal source, the angle of incidence of the signal on the panel array having the ‘best’ view can go as low as about 35°, before switchover to another panel's view is required. This means that the overall antenna gain can be as much as 5 dB below the maximum gain. For a theoretically optimum scheme, the overall gain would be equal to the maximum panel gain regardless of antenna orientation. The worst-case performance of the approach of the architecture of FIG. 3 is therefore the same as a theoretically optimum case, with one-third as many elements in each panel array. Namely, using this apparently simple ‘switchover’ concept would triple the antenna cost relative to a theoretically optimum design.
A second approach, shown diagrammatically in FIG. 4, relies upon knowledge of the spatial coordinates of the panel arrays, as well as antenna beam forming information to dynamically control the operation of a coherent combiner 50 coupled between the output of each panel and the downstream signal processing circuitry. Although this design ostensibly provides theoretically optimum performance, combining the RF outputs of multiple panel arrays also severely distorts the composite beam. Since in a practical application the panel arrays can be expected to be spaced apart by a substantial distance, which could be one hundred feet or more in a shipboard installation, the resulting pattern will contain unwanted ‘grating’ nulls. Even if some form of null compensation could be employed, there also remains the issue of the effect of the actual combiner implementation. For example, if a land vehicle upon which a terminal is mounted is moving across rough terrain, it may be necessary to compensate for the effects of structure flexing (particularly at higher signal frequencies having very short wavelengths).